Method for quantification of mineral dust in air based on optical absorption of particles concentrated by a virtual impactor and a device performing the said method

Abstract

The invention relates to a method for determination of ambient mineral dust concentration based on optical absorption of particles concentrated by a virtual impactor as well as a device performing the said method. The method comprises the following steps: Sampling air samples with particle size smaller than 1 μm (PM 1 ) and sampling air samples with particle size up to 10 μm; Concentrating the samples with particle sizes up to 10 μm with a virtual impactor; Measuring optical absorption of collected samples at least one wavelength from UV to IR spectre, preferably from 370 to 950 nm, most preferably at 370 nm; Subtracting the absorption of the samples with particle size smaller than 1 μm from the absorption of the sample concentrated by the virtual impactor.

The invention claimed is: 
     
       1. A method for quantification of ambient mineral dust concentration, said method comprising the following steps:
 sampling air samples with particle size smaller than 1 μm (PM 1 ) and sampling air samples with particle size up to 10 μm; 
 concentrating the air samples with particle sizes up to 10 μm with a virtual impactor (VI); 
 measuring optical absorption of the sampled air samples at at least one wavelength from 370 to 950 nm; 
 subtracting the absorption at the at least one wavelength from 370 to 950 nm, of the air samples with particle size smaller than 1 μm from the absorption of the sample concentrated by the virtual impactor and dividing a difference result of the subtraction with a calibration constant. 
 
     
     
       2. The method according to  claim 1 , wherein the calibration constant is a regression slope of the linear regression between absorption on y-axis and mass concentration on x-axis from chemical analysis of mineral dust particles. 
     
     
       3. The method according to  claim 2 , wherein mass concentration on x-axis from chemical analysis of mineral dust particles is based on measurements of calcium concentration using mass closure, principal component analysis (PCA) or positive matrix factorization (PMF). 
     
     
       4. The method according to  claim 1 , wherein the calibration constant is obtained by determining average concentration efficiency for particles with sizes between 1 and 10 μm and mass absorption cross section (MAC). 
     
     
       5. The method according to  claim 2 , wherein the calculation of mineral dust concentration at any time point is: 
       
         
           
             
               PP 
               = 
               
                 
                   b 
                   
                     abs 
                     , 
                     
                       VI 
                       - 
                       
                         PM 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                   
                 
                 
                   
                     db 
                     abs 
                   
                   ⁢ 
                   
                     / 
                   
                   ⁢ 
                   dPP 
                 
               
             
           
         
         wherein PP represents concentration of mineral dust, b abs, VI-PM1  represents absorption of coarse particles in the sampled air samples, while db abs /dPP represents the empirically obtained regression slope. 
       
     
     
       6. The method according to  claim 1 , wherein the optical absorption is measured at least one wavelength from UV to IR wavelength range, from 370 to 950 nm. 
     
     
       7. The method according to  claim 1 , wherein sampling and measurements of particle absorption for different size fractions are performed by any suitable absorption photometer, with at least two Aethalometers for detection of black carbon running in parallel, said Aethalometers having a PM 1  inlet and a VI inlet. 
     
     
       8. The method according to  claim 1 , wherein data for a filter loading effect are compensated with an off-line compensation using equation cBC=BC/(1-k*ATN), where cBC represents a black carbon concentration compensated for the filter loading effect, BC represents non-compensated black carbon concentration, k represents a compensation parameter and ATN represents optical attenuation of a particle laden filter. 
     
     
       9. The method according to  claim 1 , wherein mineral dust concentration data is calculated at any time point or sequence of time points. 
     
     
       10. The method according to  claim 1 , where it wherein the method is performed by a device for sampling and measuring absorbance or by a computer running a suitable software connected to measuring devices, by suitable software installed on a server or cloud. 
     
     
       11. The method according to  claim 10 , wherein the software performs at least the following calculations:
 filter loading compensation of aethalometer data; 
 calculation of a coarse particle absorption by subtracting the absorption of PM1 from the absorption of the particles from the VI; and 
 calculation of mineral dust concentration by dividing coarse particle absorption by the calibration constant. 
 
     
     
       12. A device performing the method according to  claim 1 , wherein the device comprises at least two devices for sampling airborne particles and measurement of optical absorbance at any wavelength from 370 nm to 950 nm, and a computer for calculation of mineral dust concentration in any time point or sequence of time points.